Technical Need Spark Plug Schooling

This is a really long thread.

Recently had a Willys in the shop that stopped running.
502 BBC, blown.
MSD Pro Billet magnetic pickup dist.
Older MSD 6AL box.
MSD Blaster Coil
NGK plugs, gaped to .040"

Customer stated it started running bad, went from bad to worse, to won't run at all, within about 1/2 hour, while he was trying to diagnose the problem.

So it came to me.
While cranking, sounded like it was trying to fire only one cylinder.
Both carbs have fuel, good fuel pressure, good accelerator pump shots.
Will not fire on starting fluid either.

First thing I checked was for spark.

Pulled each plug wire, tested for spark on each cylinder with a new plug, while cranking. All plug wires were firing, good hot spark.

Next, I remove all the spark plugs. All plugs appeared to be in the same condition, not super clean, but not bad nasty, no apparent physical damage. I put these plugs in the car, roughly 3 years ago, with not many miles. I cleaned and re-gapped the plugs to .040".

While the plugs were out, I did a compression test. All cylinders 165-170.

Reinstalled the plugs. Still will not run.
Found the center electrode in the dist cap was disintegrated, replaced the cap and rotor. Still will not run, sounds like its trying to fire one cylinder.

Swapped the the coil to a known good Blaster Coil. Still will not run.

Tested signal from 6AL box. OK
Tested magnetic pickup. OK

Tested ignition for spark at the coil wire by shorting magnetic pickup wires. OK, strong spark from coil to a ground.

Ohm out wires. All OK

Pulled all the plugs again and determine the only cylinder trying to fire is #8.
The other 7 plugs were clean as a whistle, like right out of the box.
They also looked dry as a popcorn fart, like fuel is not getting to those cylinders.
How could fuel not be getting to 7 cylinders???
Decided to clean #8 plug again mark it and reinstall all plugs, installing that plug in cyl#2.
Still will not run, but now cylinder #2 is the one firing.

Removed the plugs again and tested each one on a different car that had a GM HEI dist.
What I found was 5 plugs would not fire on the HEI, 3 would fire.

Then tested the plugs with an Ohm meter
All 8 plugs Ohm from the tip to the electrode with 1-2 Ohms of resistance.
2 plugs Ohm from the tip to the body of the plug, one at .880 Ohms and one at 1 Ohm, so somehow these 2 plugs are internally shorted????
Strange thing was, those two plugs that show a short, did fire on the HEI.
The 3 plugs that would not fire on the HEI, all Ohm out the same as the ones that would fire.
So 7 plugs would not fire on the MSD, and 5 plugs would not fire on the HEI.
Why the difference???????

As far as I can tell from the web, these plugs are not counterfeit NGK plugs, based on the construction and stampings. All also have the same lot number stamped on them.

Replaced the plugs with new NGKs, and now runs fine.

How could 7 spark plugs all fail at the same time??????????
What could make them fail at the same time?????????
Why did 7 not fire on the MSD and 5 not fire on the HEI????????
Could the 2 plugs that were shorted to the body, make the MSD go brain crazy?????
How could it appear to have good spark with the center electrode in cap gone??????

Spent a lot of time diagnosing bad spark plugs!!!!!!

Bill

 

I dont' know, but a few times I've got to work on a car that would not run, and replacing all the great looking spark plugs with new ones, they fired right up and ran fine.

So now I do that first, instead of wasting half a day farting with plugs that don't work.

(I think the cause of the problems here was the cap, then when the plugs were not getting juice they fouled in a way that you can't see, and then when you fixed the original problem, it would not run)

 

It was more than farting around. It was WTF, head scratching, swearing and very frustrating.
Next time, like you said, put new plugs in it!!!
Still don't understand why more appear to fire on a different ignition though.

Bill

 

I don't really understand either, but having experienced it a few times, I have decided to just do what needs to be done and not worry about it.

 

Boy that would have drove me nuts.
I too think the cap caused fouling issues that you couldn’t see.
Why some different ones fired and others not…along with the fouling issue, you may have found a few wires on each distributor to be shorting.
Try firing both engines up in a dark environment. You may have errant spark.